Allegiant Travel Co. agreed to buy Sun Country Airlines Holdings Inc. in a $1.5 billion cash-and-stock transaction, further driving consolidation in the US airline industry amid intensifying competition. Sun Country’s shareholders will receive 0.1557 shares of Allegiant common stock and$4.10 in cash per Sun Country share, the companies said Sunday. The offer is about 20% above Sun Country’s closing share price Friday, according to a release. Shares in Sun Country rose as much as 18% to $18.54 in premarke ...